1. Field of the Invention
The invention relates to a method and a gas analyzer for measuring the concentration of a gas component in a sample gas.
2. Description of the Related Art
A method or gas analyzer in the form of a laser spectrometer are known from U.S. Pat. No. 7,969,576 B1, R. Eichholz et al: “Frequency modulation spectroscopy with a THz quantum-cascade laser”, Optics Express 21(26), 32199 (2013), U.S. Pat. No. 6,351,309 B1, and H. C. Sun et al: “Combined wavelength and frequency modulation spectroscopy: a novel diagnostic tool for materials processing”, Applied Optics 32(6), 885-893 (1993).
Laser spectrometers are in particular used for optical gas analysis in process metrology. In this case, a laser diode generates light in the infrared range, which is guided along a measuring distance in a process plant or a gas cell through a process gas (sample gas), and then detected. The wavelength of the light is tuned to a specific absorption line of the respective gas component to be measured, where the laser diode samples the absorption line periodically in a wavelength-dependent manner. To this end, the laser diode is actuated by a ramp-shaped or triangular current signal (injection current) within successive sampling intervals.
With direct absorption spectroscopy (DAS), the measuring signal generated by the detector is evaluated directly, where the concentration of the gas component to be measured is determined directly from the reduction in light intensity (absorption) detected at the site of the absorption line. It is disadvantageous that the detection occurs in an extremely low-frequency range in which the gas analyzer noise (e.g., laser noise, detector noise) and the noise from the measuring distance (caused by turbulence, particles) is very high.
To avoid this problem, the injection current for the laser diode is additionally modulated sinusoidally with a predefined frequency and amplitude. With wavelength modulation spectroscopy (WMS), this modulation is performed with a frequency much lower than the full width at half maximum (FWHM) of the absorption line, typically in the kHz range. The modulation amplitude is small compared to the ramp-shaped or triangular current signal, but on the other hand high enough to ensure that the resultant spectral modulation amplitude of the laser light is greater than the full width at half maximum (FWHM) of the absorption line. The absorption line profile is not linear. As a result, high-order harmonics are also generated in the measuring signal. The measuring signal is typically demodulated at an n-th overtone, preferably the second harmonic, using phase-sensitive lock-in technology and evaluated to form a measurement result for each sampling interval.
Due to the small modulation amplitude, the detection of the n-th harmonic is directly proportional to the n-th derivative of the direct measuring signal. In the ideal case of a Lorentz-shaped absorption line, the 2f measuring signal is, for example, maximum with a modulation index of 2.2 (the modulation index is the ratio of the spectral modulation amplitude to the full width at half maximum of the sampled absorption line). The further evaluation can, for example, be achieved by fitting (curve fitting) of the profile of the demodulated measuring signal (intended curve) to be expected in the ideal case and described analytically via an approximation model to the actual profile (actual curve) thereof. One of the parameters of the approximation model is proportional to the concentration of the gas component. Consequently, the result of the evaluation, and hence the measurement result obtained, is the concentration of the gas component to be measured.
With frequency modulation spectroscopy (FMS), the injection current for the laser diode is modulated with a very high radio frequency, which is comparable to or greater than the full width at half maximum of the absorption line and can, therefore, range from tens of 10 MHz up to the GHz range. RF modulation generates side bands spaced apart from the emission frequency by an integral multiple of the modulation frequency on both sides of the emission frequency of the laser diode. The modulation index is lower than it is with WMS and selected low enough to ensure that only the two first side bands of the modulated laser light have a significant amplitude. The absorption line is investigated with these side bands. As is also the case with WMS, in addition to the modulation of the wavelength, FMS also results in modulation of the intensity of the laser light, where wavelength modulation is dominant and the amplitude modulation only provides a small contribution to the measuring signal. Therefore, laser diodes to be used for FMS (for example, lead salt lasers or quantum cascade lasers) have to be suitable for wavelength modulation at the radio frequencies described and the detectors must also have a very large bandwidth. This means the components and structure of an FM spectrometer are very expensive and complex. In order to use detectors with a lower bandwidth, with two-tone FMS, the laser diode is modulated with two radio frequencies that are very close to one another and detection is performed at the beat frequency.
DAS, WMS and FMS have specific advantages and disadvantages. WMS and FMS are in particular advantageous for the measurement of low concentrations because they are more efficient at filtering noise out of the measuring signal. However, at higher concentrations, the approximations required for the evaluation of the measuring signal are increasingly inaccurate thus resulting in a greater measuring error. Moreover, FMS is very expensive and complex. The reverse applies with DAS; due to the greater sensitivity to noise, the measuring error is higher at low concentrations. However, since no approximate description of the absorption line is required, the measuring accuracy improves as the concentration increases because the effective signal is stronger. It is only at very high concentrations (absorption saturation) that the measuring method once again becomes more inaccurate.
The method or gas analyzer known from the aforementioned U.S. Pat. No. 7,969,576 B1 work based on WMS where, in addition to the n-th harmonic of the measuring signal, its first harmonic, i.e., the fundamental frequency of the modulation, is evaluated to normalize the measurement result.
The detection limit and limit of determination for the measurement of the concentration of the gas component is restricted by noise, which superimposes the measuring signal and is primarily composed of the noise of the gas analyzer (laser noise, detector noise) and the noise from the measuring distance (caused by turbulence, particles). The longer the measuring distance, the greater the absorption and the measuring signal obtained. The measurement of small concentrations requires a sufficiently long measuring distance.